(1) Field of the Invention
The present invention relates to a fluorescent lamp, a backlight apparatus including the fluorescent lamp, and a manufacturing method of the fluorescent lamp.
(2) Description of the Related Art
In recent years, various types of fluorescent lamps have been developed for use. The fluorescent lamps contain mercury in a glass bulb constituting an arc tube, where mercury radiates ultraviolet light when it obtains energy from electrons. However, reaction of the mercury with the glass bulb (hereinafter, the reaction is referred to as “mercury reaction”) may cause a defect such as the change of color of the glass bulb or the consumption of the mercury that brings a short life of the lamp.
As a result, technologies for restricting the mercury reaction by forming a protection layer between the glass bulb and the phosphor layer have been proposed (see for example, Japanese Laid-Open Patent Application No. 1-112651 and Japanese Laid-Open Patent Application No. 2003-123691). FIG. 1 is an enlarged photo of a cross section of a conventional arc tube taken along a plane that includes the tube axis. FIG. 1 shows that a protection layer 102 and a phosphor layer 104 are stacked on a glass bulb 100.
Meanwhile, the protection layer 102 is composed of particles of a metal oxide, and gaps “A” exist between the particles. The inventors of the present invention found that when the gaps A exist in the protection layer 102, visible light is reflected diffusedly at interfaces that have different refractive indexes, thus decreasing the luminous flux of the arc tube. Also, from the viewpoint of restricting the mercury reaction, it is necessary to make the protection layer 102 thick enough. Here, if the protection layer 102 is made thick for restricting the mercury reaction, the luminous flux decreases due to the above-mentioned diffused reflection. Conversely, if the protection layer 102 is made thin enough to ensure the luminous flux, the mercury reaction is not restricted well.
Considering a method for solving the above-mentioned problems, the inventors of the present invention first came up with an idea of enhancing the bulk density of the metal oxide particles in the protection layer. With this method, it is possible to reduce the gaps between the particles in the protection layer 102 and restrict the diffused reflection, and to restrict the mercury reaction without reducing the luminous flux since it can make the layer more thick than in the conventional technologies.
The inventors of the present invention manufactured pre-production samples of the fluorescent lamps in which the bulk density of the particles in the protection layer 102 has been increased compared with the conventional technologies, and confirmed with the pre-production samples that the luminous flux does not decrease if the thickness of the protection layer 102 is increased to a certain extent.
However, the inventors found a new problem in the pre-production samples of fluorescent lamps that the difference in chromaticity is observed for the entire arc tube since the phosphors are distributed unevenly.